Line data    Source code

       1             : /*
       2             :  * Copyright 2015 Google Inc.
       3             :  *
       4             :  * Use of this source code is governed by a BSD-style license that can be
       5             :  * found in the LICENSE file.
       6             :  */
       7             : 
       8             : #include "GrDistanceFieldAdjustTable.h"
       9             : 
      10             : #include "SkScalerContext.h"
      11             : 
      12             : SkDEBUGCODE(static const int kExpectedDistanceAdjustTableSize = 8;)
      13             : 
      14           0 : SkScalar* build_distance_adjust_table(SkScalar paintGamma, SkScalar deviceGamma) {
      15             :     // This is used for an approximation of the mask gamma hack, used by raster and bitmap
      16             :     // text. The mask gamma hack is based off of guessing what the blend color is going to
      17             :     // be, and adjusting the mask so that when run through the linear blend will
      18             :     // produce the value closest to the desired result. However, in practice this means
      19             :     // that the 'adjusted' mask is just increasing or decreasing the coverage of
      20             :     // the mask depending on what it is thought it will blit against. For black (on
      21             :     // assumed white) this means that coverages are decreased (on a curve). For white (on
      22             :     // assumed black) this means that coverages are increased (on a a curve). At
      23             :     // middle (perceptual) gray (which could be blit against anything) the coverages
      24             :     // remain the same.
      25             :     //
      26             :     // The idea here is that instead of determining the initial (real) coverage and
      27             :     // then adjusting that coverage, we determine an adjusted coverage directly by
      28             :     // essentially manipulating the geometry (in this case, the distance to the glyph
      29             :     // edge). So for black (on assumed white) this thins a bit; for white (on
      30             :     // assumed black) this fake bolds the geometry a bit.
      31             :     //
      32             :     // The distance adjustment is calculated by determining the actual coverage value which
      33             :     // when fed into in the mask gamma table gives us an 'adjusted coverage' value of 0.5. This
      34             :     // actual coverage value (assuming it's between 0 and 1) corresponds to a distance from the
      35             :     // actual edge. So by subtracting this distance adjustment and computing without the
      36             :     // the coverage adjustment we should get 0.5 coverage at the same point.
      37             :     //
      38             :     // This has several implications:
      39             :     //     For non-gray lcd smoothed text, each subpixel essentially is using a
      40             :     //     slightly different geometry.
      41             :     //
      42             :     //     For black (on assumed white) this may not cover some pixels which were
      43             :     //     previously covered; however those pixels would have been only slightly
      44             :     //     covered and that slight coverage would have been decreased anyway. Also, some pixels
      45             :     //     which were previously fully covered may no longer be fully covered.
      46             :     //
      47             :     //     For white (on assumed black) this may cover some pixels which weren't
      48             :     //     previously covered at all.
      49             : 
      50             :     int width, height;
      51             :     size_t size;
      52             : 
      53             : #ifdef SK_GAMMA_CONTRAST
      54             :     SkScalar contrast = SK_GAMMA_CONTRAST;
      55             : #else
      56           0 :     SkScalar contrast = 0.5f;
      57             : #endif
      58             : 
      59             :     size = SkScalerContext::GetGammaLUTSize(contrast, paintGamma, deviceGamma,
      60           0 :         &width, &height);
      61             : 
      62           0 :     SkASSERT(kExpectedDistanceAdjustTableSize == height);
      63           0 :     SkScalar* table = new SkScalar[height];
      64             : 
      65           0 :     SkAutoTArray<uint8_t> data((int)size);
      66           0 :     SkScalerContext::GetGammaLUTData(contrast, paintGamma, deviceGamma, data.get());
      67             : 
      68             :     // find the inverse points where we cross 0.5
      69             :     // binsearch might be better, but we only need to do this once on creation
      70           0 :     for (int row = 0; row < height; ++row) {
      71           0 :         uint8_t* rowPtr = data.get() + row*width;
      72           0 :         for (int col = 0; col < width - 1; ++col) {
      73           0 :             if (rowPtr[col] <= 127 && rowPtr[col + 1] >= 128) {
      74             :                 // compute point where a mask value will give us a result of 0.5
      75           0 :                 float interp = (127.5f - rowPtr[col]) / (rowPtr[col + 1] - rowPtr[col]);
      76           0 :                 float borderAlpha = (col + interp) / 255.f;
      77             : 
      78             :                 // compute t value for that alpha
      79             :                 // this is an approximate inverse for smoothstep()
      80           0 :                 float t = borderAlpha*(borderAlpha*(4.0f*borderAlpha - 6.0f) + 5.0f) / 3.0f;
      81             : 
      82             :                 // compute distance which gives us that t value
      83           0 :                 const float kDistanceFieldAAFactor = 0.65f; // should match SK_DistanceFieldAAFactor
      84           0 :                 float d = 2.0f*kDistanceFieldAAFactor*t - kDistanceFieldAAFactor;
      85             : 
      86           0 :                 table[row] = d;
      87           0 :                 break;
      88             :             }
      89             :         }
      90             :     }
      91             : 
      92           0 :     return table;
      93             : }
      94             : 
      95           0 : void GrDistanceFieldAdjustTable::buildDistanceAdjustTables() {
      96           0 :     fTable = build_distance_adjust_table(SK_GAMMA_EXPONENT, SK_GAMMA_EXPONENT);
      97           0 :     fGammaCorrectTable = build_distance_adjust_table(SK_Scalar1, SK_Scalar1);
      98           0 : }